In vehicles such as automobiles and the like, a transmission is interposed between an internal combustion engine and driving wheels. The transmission changes the driving force which is transferred from the internal combustion engine to the driving wheels and the running velocity in accordance with running conditions of the vehicle. Because such running conditions change in a wide range, the transmission makes the performance of the internal combustion engine sufficiently effective.
In a known gear type transmission, by selectively switching engaging states of a plurality of stages of gear trains, a gear ratio is changed step by step and the driving force is transferred. In a known continuously variable transmission, each of a driving side pulley and a driven side pulley has a fixed pulley member fixed to a rotary shaft and a movable pulley member which is attached to the rotary shaft so that it can come into contact with and move away from the fixed pulley member. A groove width formed between the fixed and movable pulley members of each of the driving side and driven side pulleys is decreased and increased, whereby the rotational radius of a belt reeved between both of the pulleys is respectively increased and decreased. The belt ratio is thus continuously changed, thereby transferring the driving force.
A control method for such a continuously variable transmission is disclosed in JP-A-64-44338. According to the method disclosed in the above document, in a control section which receives detection signals of various operating modes, if it is determined that the operating mode is a holding mode, then a hydraulic circuit is operated and a clutch pressure is controlled to a predetermined pressure, thereby achieving a clutch engagement state which is expected by the driver.
On the other hand, in such a control method for the continuously variable transmission, when the selected operating mode is a normal starting mode, an actual engine rotational speed is fed back and compared to an objective engine rotational speed which is linearly determined based on a throttle opening degree. The difference between the objective engine rotational speed and the actual engine rotational speed is calculated. A feed forward amount is calculated from an engine torque value which is determined based on the throttle opening degree, and an objective clutch pressure for the hydraulic clutch is calculated from (1) the aforementioned difference between the objective and actual engine rotational speeds, and (2) the feed forward amount.
When the engine is started at a low temperature, for the purpose of warming-up the engine and maintaining the normal operable state of the warmed engine, the throttle is normally opened slightly more (for instance, by about 2 to 20%) than at ordinary engine operating temperature, which engine temperature is determined in accordance with the temperature of the engine cooling water. The engine rotational speed is thereby raised to a fast idling rotational speed (for instance, 1500 to 2500 r.p.m.) at low engine temperatures. On the other hand, when the driver depresses the accelerator pedal upon starting the vehicle in motion, the hydraulic clutch of the continuously variable transmission is controlled in the normal starting mode.
However, the objective idling rotational speed in the normal starting mode has been predetermined based on ordinary engine operating temperature, and is therefore lower than the aforementioned fast idling speed. Therefore, after the engine is started at a low temperature, at the start of vehicle motion, the objective idling rotational speed corresponds to a throttle opening degree of small (warmed-up) value, and the clutch pressure is therefore controlled so as to reduce the engine rotational speed below the fast idling rotational speed. Thus, the warming-up of the engine after the start of the engine at a low temperature does not progress, and because the normal operable state of the warmed engine is insufficient at the start of vehicle motion when the engine is cold, the engine rotational speed decreases. In the worst case, the vehicle stops because of engine stall, or an abnormal over rotational speed is caused so that a shock occurs at the start of vehicle motion, and the vehicle cannot be smoothly started. As a result, the feeling associated with the start of vehicle motion is deteriorated.
It is therefore an object of the invention to provide a method of controlling the rotational speed of a continuously variable transmission, in which, after the engine is started at a low temperature, a decreased or abnormally high engine rotational speed at the start of vehicle motion can be avoided. Thus, the vehicle can be smoothly started, and the feeling of starting vehicle motion can be improved.
According to the present invention, in a method of controlling rotational speed of a continuously variable transmission in which each of a driving side pulley and a driven side pulley has a fixed pulley member and a movable pulley member which is attached to the fixed pulley member so that it can come into contact with and be moved away from the fixed pulley member, and in which the groove width between both of the fixed and movable pulley members of each of the pulleys is decreased and increased so that the rotational radius of a belt reeved between both of the pulleys is respectively increased and decreased, thereby continuously changing the belt ratio, the continuously variable transmission is characterized in that it includes a hydraulic clutch which engages with and disengages from the transmission in response to a clutch pressure to connect (and disconnect) a driving force of the transmission to (and from) a vehicle drive train. There is provided a control section for calculating an objective clutch pressure of the hydraulic clutch from (1) an engine torque value which is determined based on a throttle opening degree, and (2) an objective engine rotational speed. An actual clutch pressure in each of various control modes is set to the objective clutch pressure by the control section, and in the case where the selected control mode is a normal starting mode, if predetermined correction executing conditions are satisfied, then the control section corrects the objective engine rotational speed based on the oil temperature of the transmission.
The correction executing conditions may be: the oil temperature is equal to or lower than a predetermined temperature; the elapsed time after the start of the engine lies within a predetermined time; the throttle opening degree is equal to or smaller than a predetermined opening degree; the temperature of the engine cooling water is equal to or lower than a predetermined temperature; and the like. In the normal starting mode, if at least one of the correction executing conditions is satisfied, then the final objective engine rotational speed is calculated in accordance with the oil temperature. The objective clutch pressure is calculated from the engine torque value, which is decided by the throttle opening degree, and from the final objective engine rotational speed. The clutch pressure is controlled so that the actual clutch pressure in the normal starting mode is set to the objective clutch pressure, thereby avoiding the undesirable situation wherein, after the engine is started at a low temperature, the clutch pressure is controlled so as to reduce the engine rotational speed below the fast idling rotational speed at the start of vehicle motion. Abnormally high rotational speed is also avoided.